I. Field of the Invention
The current invention relates to communication. More particularly, the present invention relates to a novel method and apparatus for recovery of particular bits of a corrupted frame.
II. Description of the Related Art
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting an information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the channel bandwidth. At the destination station the original message signal is replicated from a version of the modulated carrier received subsequent to propagation over the channel. Such replication is generally achieved by using an inverse of the modulation process employed by the origination station.
Furthermore, the conversion is selected in accordance with additional characteristics of the channel, including, but not being limited to, signal-to-noise ratio, fading, time variance, and others known to one skilled in the art. Thus, transmission of an information signal over a wireless communication channel will require different consideration than transmission over a wire-like channel, e.g., coaxial cable, optical cable, and others known to one skilled in the art.
Modulation also facilitates multiple-access, i.e., simultaneous transmission, of several signals over a common channel. Multiple-access communication systems often include a plurality of remote subscriber units requiring intermittent service of relatively short duration rather than continuous access to the communication channel.
There are several multiple-access communication system techniques, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), and amplitude modulation (AM) schemes such as amplitude companded single sideband known in the art. Another type of multiple-access spread spectrum system is a code division multiple-access (CDMA) modulation system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The CDMA system supports voice and data communication between users over a terrestrial link. The use of CDMA techniques in a multiple-access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention and incorporated herein by reference.
In the above-referenced U.S. Pat. No. 4,901,307, a multiple-access technique is disclosed allowing a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple-access techniques.
In general, the transmitted information signal is divided into a number of “frames,” each of which includes a specified number of information bits and a number of quality metric bits. Each frame is processed in accordance with a selected modulation scheme, and transmitted over a communication channel. At the destination station, the frame is extracted from the communication channel by demodulation. In order to ascertain integrity of information in the extracted signal, the information bits in the frame are protected by a quality metric derived from the information bits. Such a quality metric may be a parity bit, a cyclic redundancy check (CRC), or any other quality metric known to one skilled in the art. Upon extraction of the signal from the received frame, a quality metric is determined from the extracted information bits and compared with the extracted quality metric. If the two quality metrics match, the frame is considered correctly received. Otherwise, the frame is declared erased.
The above-described integrity check works well when all the information bits in the frame are equally important. However, certain applications may use frames with bits grouped into blocks of different importance. An example of a frame of this structure is disclosed in co-pending provisional application Ser. No. 60/175,371, entitled “Accommodating the WCDMA AMR Data Rates in IS-2000 MC,” filed Jan. 10, 2000, assigned to the assignee of the present invention, and incorporated herein by reference. The Adaptive Multi-Rate (AMR) speech coder groups the information bits into three classes called class A, class B, and class C. In a Wideband Code Division Multiple-access (WCDMA) system, each class of bits is sent on a different transport channel with possibly different coding and rate matching. The Class A bits are the most important bits, then the Class B bits, and finally the Class C bits. A WCDMA approach uses an 8-bit CRC and a tailed-off convolutional coding for the Class A bits, no CRC and tailed-off convolutional coding for the Class B bits, and no CRC or convolutional coding for the Class C bits. The Telecommunication Industry Association (TIA) Industry Standard-2000 Multi-Carrier (IS-2000 MC) accommodates the AMR speech coder by forming a single frame comprising all three classes A, B, and C with reverse ordering of the AMR information bit classes so that the last (least important) bit class (Class C) is sent first. Because the flexible-rate puncturing starts from the first encoded and repeated symbols and stops after the necessary number of symbols are punctured, the bits located at the end of the frame are more reliable. Puncturing is a technique that affects bit, or bits, legitimately belonging at certain location in a frame. Thus, for example, in power control puncturing, information bits at certain locations are replaced by power control bits. In another example, interleaving may generate bits exceeding a frame length, and the excess bits are discarded. The frame is encoded by a single, tailed-off convolutional code. A single CRC with a length depending on the number of information bits is determined in accordance with all the information bits. Such a frame is depicted in FIG. 1. FIG. 1 shows a frame structure 100, in which the information bits within the frame are classified as class A 106, class B 104, and class C 102. The different classes are of different importance. In accordance with the method known in the art, all the information bits are protected by a single CRC 108. The frame also contains tail bits 110. The tail bits 110 do not carry any information, and are all zeros. The tail bits 110 are used to initialize an encoder (not shown) for the next frame. However, if the CRC integrity check fails, all the information bits, regardless of their importance, are unrecoverable.
The above description uses a wireless communication system as a particular example of a frame with bits of different importance. One skilled in the art will appreciate that this is for explanatory purposes only because the problem of recovery of particular bits of an erased frame is inherent in any communication system.
Because it may be desirable to recover the relatively more important block, or blocks, of bits from an erased frame, there exists a need in the art for an integrity check mechanism allowing recovery of the block, or blocks, of bits from an erased frame.